1. Field of the Invention
The present invention relates to semiconductor devices including a drive circuit as formed using a semiconductor element with a semiconductor film as an active layer (a semiconductor layer including a channel formation region, a source region and a drain region) and also to a manufacturing method of the same. Note that typical examples of the semiconductor element are transistors including, although not limited to, field effect transistors such as metal oxide semiconductor (MOS) transistors and thin-film transistors (TFTs). In addition, the present invention relates in particular to large size display devices (more than 20 inches) wherein resistance of wiring lead becomes a problem in supplying signal, and also to a fabrication method thereof.
2. Description of the Related Art
To realize reduction in weight and power consumption of portable information equipment such as a mobile telephone and a notebook PC, small-size or middle-size liquid crystal display devices have generally been used as display units thereof due to merits of such liquid crystal display devices.
Further, it is becoming a more active trend to enlarge the market of the liquid crystal display device up to a television (TV) and also to design a TV at home by using the liquid crystal display device in place of a traditional cathode ray tube (CRT). However, it is necessary to simultaneously satisfy higher fidelity and higher brightness in addition to the enlargement to realize a liquid crystal TV.
Further, with an increase in size of a display device, the number and length of wirings and resistivities of the wirings increase accordingly. An increase in wiring resistance would cause a delay in transmitting signals toward lead terminate ends to in turn badly affect the resultant displays. Therefore, a technique for reducing the wiring resistance becomes inevitable in order to widely spread the liquid crystal display devices to ordinary homes.
So far, as the technique for reducing wiring resistance, a method of widening line widths of the wirings and increasing the film thickness of the wirings have been considered, and actually, it has been realized to reducing the wiring resistance. However, the former is encountered with an enlargement of element areas in a pixel portion due to widening of the lead widths to result in decreasing an aperture ratio and to make it impossible to obtain higher brightness required. Alternatively, with the latter method, the increase in film thickness of wirings results in an increase in difference between a layer formed under the wirings and a layer formed on the wirings to cause production yields to decrease due to problems such as disconnection as a result of reduction of coating properties in fabricating insulating films and metal films for electrodes.
Additionally, although there is a method of using aluminum (Al) or copper (Cu) as materials with low resistance for wirings, which suffer from defects of less corrosion resistivity and less thermal resistivity. It occurs as problems that projections such as hillocks and whiskers or the like are formed by thermal treatment, and that aluminum atoms behave to diffuse into a channel formation region to result in operation defects of TFTs or degradation in TFT characteristics. In this way, while it is not easy to form a suitable gate electrode of a TFT by using the above-noted metal materials, no materials are less in resistance than aluminum (Al) or copper (Cu). The above serves as a problem against producing a liquid crystal display device with a large screen.
In this way, the above-stated problem becomes more appreciable with increasing performance required for a semiconductor device which has a plurality of integrated circuits such as an active matrix type liquid crystal display devices.
The present invention is performed in view of the problem stated above, and the object is to provide, in a semiconductor device typically represented by an active-matrix type liquid crystal display device which has a circuit using semiconductor elements, a technique for realizing low resistivity of wirings required for enlargement and higher precision without increasing the number of steps in the manufacturing process, and further provide a method for fabricating a semiconductor device at low temperatures (temperatures lower than or equal to the distortion point of a glass substrate) to use glass substrates with a low cost.
The invention has the following structure. A conductive film containing W as its main component is used for a first layer to form a gate electrode in order to prevent aluminum of the gate electrode from transpiring and diffusing into a channel formation region with a film which contains Al as its main component and is low in resistance as a second layer and a film which contains Ti as its main component, to fabricate the gate electrode with a laminate structure of the above materials by using an apparatus which is capable of performing etching treatment at high speeds.
Additionally it is required that a TFT disposed in each of a variety of types of circuits be fabricated as a TFT in accordance with the function of the circuit. For example, it is desirable that the TFT provided in a driver circuit required to achieve high-speed operations be designed to have a specific structure with great emphasis on an increase in operation speed and at the same time on suppression of any possible degradation as an appreciable problem due to hot carrier injection. It is known as such structure that an LDD region as provided between a channel formation region and a drain region has a concentration in order that the concentration of an impurity element gradually gets higher toward the drain region. With the structure, the effect of relaxing electric field becomes more remarkable in a depletion layer adjacent to the drain region.
In order to form the LDD region with the above-noted concentration gradient of the impurity element, the present invention use a method of accelerating by electric fields an ionized impurity element for giving one conductivity type to pass through a gate insulating film and then add into a semiconductor layer. In addition, with the present invention, a gate electrode with a tapered shape is formed to have a thickness gradually increasing from an end portion toward inside by etching, and it is considered that the impurity element is somewhat added to the semiconductor layer through the tapered shape. In the present invention, the LDD region is formed in order that the concentration of the impurity element changes in a direction along the channel length of a TFT, without increasing the number of steps (without increasing the number of masks).